During surgery, anesthetized patients are usually intubated. Measurement of respiratory gases is desirable when a patient is mechanically intubated through an endo-tracheal tube. An analysis of the inhaled and exhaled gas mixture provides information about the patient's ventilation.
Carbon dioxide (CO.sub.2), nitrous oxide (N.sub.2 O) and the anesthetic agent are the constituent gases of most interest in measuring respiratory gas streams.
It is well known that CO.sub.2 in the bloodstream equilibrates rapidly with CO.sub.2 in the lungs. Hence, the partial pressure of the CO.sub.2 in the lungs approaches the amount in the blood during each breath. Accordingly, the CO.sub.2 content at breath's end, termed end-tidal CO.sub.2, is a good indication of the blood CO.sub.2 level.
Abnormally high end-tidal CO.sub.2 values indicate that an insufficient amount of CO.sub.2 is being transported away from the bloodstream through the lungs, i.e., inadequate ventilation. Conversely, abnormally low end-tidal CO.sub.2 values indicate poor blood flow to the tissues, inadequate CO.sub.2 transport through the lungs, or excessive ventilation.
Mass spectrometers are used for measuring the partial pressure of respiratory gases in, for example, operating room suites in which one spectrometer is shared by many rooms. Mass spectrometers have the advantage of measuring a multiplicity of gases; however, the disadvantages are their cost, maintenance and calibration requirements, slow response time, and noncontinuous measurement.
Gas analyzers using non-dispersive infrared spectrophotometry are also used for partial pressure gas measurement. While these analyzers are less expensive than mass spectrometers and continuously measure partial gas pressure, their disadvantages are poor response time and difficulty in calibration.
Prior art non-dispersive infrared gas analyzers include features for making CO.sub.2 and N.sub.2 O cross channel detection, temperature, and collision broadening corrections to their partial gas pressure measurements. Some of these corrections are made automatically by the analyzers while others are made manually by the operator.
Non-dispersive infrared gas analyzers generally have two configurations. The first, and most common, is the sampling or side-stream type. This type diverts a portion of the patient's respiratory gas flow through a sample tube to the infrared analyzer.
The second type mounts on the patient's airway and uses a portion of the airway as the sample chamber. This type is frequently occluded by the mucus and moisture in the patient's airway and its bulk on the airway can affect the patient's breathing.
Both infrared gas analyzer configurations are characterized by small absorption levels by the constituent gases which lead to small signals and stability problems.
Increasing the analyzer's sample chamber size improves the small signal and stability problems; however, it also increases the response time. Increasing the gas flow rate through the analyzer improves the response time, but occlusions are more frequent and the patient's normal ventilation volume is impaired.
In this regard, neonates require sample flow rates equal to or less than 50 cc/minute. However, neonates also require the analyzer's response time to be compatible with breath rates well in excess of 60 breaths per minute. This condition equates to a response time of less than 100 milliseconds.
Another disadvantage of infrared gas analyzers is that they require frequent calibration for proper operation. Factors affecting calibration of the optical bench portion of a gas analyzer include manufacturing tolerances relating to the sample cell dimensions (particularly thickness); the brightness of the infrared sources and sensitivity of the photodetectors; temperature; barometric pressure; and the accumulation of dirt or moisture in the optical bench gas pathways.
Changes in the optics and electronic circuitry over time require recalibration of infrared gas analyzers. Careful construction of the optics and electronic circuitry minimizes the number of calibration adjustments needed and the period between recalibration. Hence, interchangeability of the optical bench of an analyzer has not heretofore been practical because of the need for recalibration when the optical bench is connected to the analyzer.
Calibration of infrared gas analyzers is accomplished by various electronic circuit adjustments to correct for variations in sample chamber geometry as well as variations and drift of various sensing components.
Calibration usually requires taking the analyzer out of service and passing standard gases through it, in the presence of which the various adjustments are made. Another calibration method is to make a "zero gas" reading for the optical bench and adjust the analyzer's amplifier so that the analyzer's output actually reads zero. A still further method uses a reference cell filled with a non-absorbing gas or a reference filter having a wavelength at which no absorption takes place to stabilize the zero setting of the analyzer.
Prior art non-dispersive infrared gas analyzers also include some automatic calibration features. However, further operator controlled calibration procedures are required before the analyzers are ready for use.
The present invention overcomes these and other problems of prior infrared gas analyzers as will be set forth in the remainder of the specification.